# How does aerogel attain its enormous heat isolation properties?

In a text about properties of aerogel I read some interesting facts - clothes with aerogel proved to be impractical, being far too warm to wear. If you isolated a house with aerogel and lit a candle in one of the rooms, after a few days the house would become too hot to live in, just accumulating the candle heat.

What properties (thermal mechanics?) of aerogel allow it to be this good a thermal isolator?

Would a sealed box made of aerogel work just as well maintaining the temperature of enclosed (solid) items in void - with no air inside the box?

Two ideas are especially to the heat isolation properties of aerogels: (i) aerogels are mostly gas and (ii) the Knudsen Effect.

Clearly, heat isolation is achieved by blocking heat transfer paths: these are the paths of convection and conduction at the temperatures of interest (radiation also becomes important at higher temperature). Any material barrier blocks direct convection by moving liquid or gas, so a wall's thermal isolation properties are set by how low its conductivity is.

An aerogel is made up mostly of gas, so its conductivity is naturally very low. However, many aerogels have conductivities significantly below that of the gas that mostly composes them. The gas cavities in aerogel have diameters of significantly below the mean free path of the gas in question over the operating temperature range of the aerogel, thus greatly hindering the gas's conductivity mechanisms.

• ...so, in void, convection and conduction become nearly non-issue; how would it fare against radiation?
– SF.
Nov 17 '14 at 11:44

Simple answer: tenuity. Thus, vacuum is the best isolator (remember double window - attempt to save the heat in the house or vacuum flask, thermos) cause it hurdles to transfer kinetic energy via particles, i.e. the more space between particles the less collision between them the less heat transfer then. These principles mostly underlie aerogel class materials.

The space between particles could not be simply measured as direct line between them. Imagine, you could twist and cycle most of the particles to prevent colliding - they will vibrating in more little areas but with more large distances (if we unbend the curl) and it would result in a compact and very heat isolative material.

• Could you at least provide a definition (within the scientific world) of "tenuity" ? Nov 17 '14 at 13:26
• Maybe I needed to use more common word rarefaction. I ment tenuity as a measure of distance between particles made up the material.
– rook
Nov 17 '14 at 13:52